1. Field of the Invention
The present invention relates to a control unit for a three-phase AC motor.
2. Description of the Related Art
Conventionally, in the three-phase AC motor, a sine wave current of a fundamental frequency has been caused to flow through each phase to drive the motor. The maximum torque that the motor is capable of outputting is determined by a peak current value of the sine wave current for flowing through each phase, that is, magnitude of amplitude.
In order to improve voltage utilization efficiency, there is known a control method for superimposing a voltage command having a frequency which is three times as high as a fundamental frequency on a voltage command of the fundamental frequency to each phase into the voltage command, creating a PWM signal and driving an inverter (See Japanese Patent Application Laid-Open Nos. 6-133558 and 11-69899).
In the invention described in the above-described patent document, the voltage utilization efficiency is improved by superimposing, on voltage of the fundamental sine wave, voltage having a frequency which is three times as high as the frequency of the fundamental sine wave to obtain each phase voltage command. A current flowing through each phase is sine wave, the maximum current value is a peak value of the sine wave, and the torque cannot be increased by any other than this peak value, that is, enlargement of the amplitude.